Since the discovery of carbon nanotubes (CNTs) (Ijima, S, Nature, 1991, 354, 56), their remarkable mechanical, electrical and thermal properties have been studied. Single-walled nanotubes (SWNTs) are perfect examples of a near one-dimensional, tubular macromolecule. This all carbon tube forms a continuous pi-electron structure with no edges or defects. The aromatic nature of the graphene surface contributes to all the molecular stabilization that gives CNTs their unique properties. In combination with their chemistry, their tubular structure gives them approximately 7 times the strength of steel with much less weight. These extraordinary mechanical properties have inspired continued interest in using CNT as filler additives in polymeric composites for obtaining ultra-light structural reinforcement with the potential for multi-functional applications.
Despite the promise of stronger and lighter composites, CNTs have not made the leap into mainstream manufacturing other than a few niche examples. One main reason for the lack of full production CNT-based composites is cost. This is typically not an issue when the ultimate strength is concerned. A more fundamental issue is the inherent problems with both dispersion and adhesion of the CNTs with the host resin. Improving the adhesion between the polymer matrix and CNT improves load transfer from the polymer to the nanotubes. This commutes stress points that can undermine the polymer by introduction of cracks that could propagate through the material. There are numerous examples of functionalizing the CNTs to promote adhesion in composite materials. However, there is still a lack of methods for proper dispersion that dominates the physical load transfers.
Carbon nanotubes have sp2 carbon-carbon double bonds oriented along the tube axis. These sp2 pi-bonds create an extended network of overlapping electron density creating the excellent electrical and thermal conductivity characteristics of carbon nanotubes. Likewise, this bond structure produces a very strong material with extremely high strength to weight ratio. The drawback to this structure is the extended pi-bond structure causes the nanotubes to have a very high affinity for other nanotubes. This causes them to stick together in ropes or bundles. The Van der Waals forces that hold them together are very strong and make it difficult to separate the CNTs. These ropes or bundles present problems for ultimate strength in a host matrix. Nanotubes can easily slip along their longitudinal axis. The ability to slide against each other creates noticeable defects in a composite structure. It is imperative to have highly dispersed individual CNTs when maximum strength is required. There are several methods to disperse CNTs into a host matrix such as a polymer resin. Most of these methods are based upon chemical functionalization.
The pi-bonds on CNTs provide surface reaction sites to derivatize the nanotubes. Taking advantage of well known organic chemistry reactions involving alkenes or aromatic compounds, chemical end groups can be covalently bonded to the surface to engineer the CNT for a particular application. For example, to increase solubility in a polar solvent one could add an acid or hydroxyl terminated end groups. The type and degree of functionalization can be used to provide exfoliation of the CNT bundles and increased interaction with the host matrix. When attached to the side walls of the CNTs, the functional groups provide a charged surface that prevents the CNTs from sticking together, thus providing stability in solution. Functional groups can also be used to increase the interaction of the CNT in a host matrix, for example is structural resin. However, this functionalization comes at a price. Increasing the degree of functionalization will disturb the extended □-structure that gives the CNTs their fantastic properties. In short, covalently attaching side groups weakens the structural properties of CNTs. There have even been calculations that indicate that non-functionalized CNTs have a higher Young's modulus compared with functionalized CNTs when inserted into a polyethylene composite. (Odegard, G. M. et al., AIAA Journal, 2005, (43) 1828).
To maximize the physical characteristics of CNTs in a matrix, the CNTs must exist as individual tubes. The most common method for deaggregation of CNTs is ultrasonication of a CNT solution. (J. Sandler, et al. Polymer, (1999) 40, 5967). Starting with powdered forms of CNTs suspended in a solvent, high enemy sonic waves vibrate the tubes with enough energy to separate from bundles to individual tubes. Without some method of stabilization, the tubes will reagglomerate as soon as the sonication (energy input) is stopped. There needs to be some energy barrier to prevent the CNTs from reagglomerating. The best method for physical separation typically involves sonication of the CNTs in solution with a stabilizing agent such as a polymer or surfactant. The sonic energy serves to deagglomerate the CNTs while the stabilizing agent adheres to the surface preventing them from sticking back together. They further provide a steric barrier for agglomeration. These methods will not work for high strength composites since the stabilizing agents typically become impurities producing defect sites in the high-strength resin. Electric field manipulation, ball-milling and polymer wrapping are other physical techniques that have been proposed by a few researchers for breaking up the agglomerates. These typically do not provide enough energy to overcome the Van der Waals forces that hold the tubes together. This leads us to chemical techniques for dispersion.
Chemically functionalized CNTs are more efficient as reinforcing agents in polymers than pristine CNTs because functionalized nanotubes typically are more stable and have a more uniform dispersion in organic solvents. The secondary effect is that these functional groups attached to the surface of the CNT can be designed to establish covalent linkages with the polymer matrix during a cross-linking process. Under such scheme, load transfer from matrix to CNT under an external force is significantly increased from the increased dispersion and the chemical functionalization resulting in improved properties. Chemical bonding can make the CNTs a structural part of the polymer matrix rather than just a mixed in additive.
Oxidation through acidification is one of the widely used functionalization procedures. Oxygen-containing functional groups including hydroxyls, carbonyls, esters, ethers and importantly carboxylic acids have been identified in oxidized CNTs. These groups are commonly attached through an acidic purification step during synthesis of the carbon nanotubes. Placement of these groups is on the ends of the CNTs and to a lesser degree the side-walls. When attached to the sidewalls these oxygen-containing groups remarkably improve the exfoliation and interfacial bonding in a polymer matrix. The drawback of this approach is that the oxidation brings serious damage to the sidewall and even disrupts the and structure. These damaged defect sites reduce strength in the CNTs and a resulting polymer matrix. Solvent-free chemical oxidation techniques proposed at Rice University (Tour et al., Journal of Physical Chemistry A, 2004, 108, 51) successfully improved the solubility of CNTs in organic solution but severely damaged the sidewalls of the tubes. This prevents their use in structural composites. Each time a functional group is placed on the sidewall of is CNT it increases its solubility or stability in as matrix while drastically reducing its physical strength. There are no chemical techniques available for covalently modifying the CNTs without disrupting its chemical, electrical and structural integrity.
Functionalized CNTs are mainly based on the carboxylic acid groups formed by oxidation of CNTs using strong acids (J. Chen, et al Science 1998, 282; 95; W. Zhao, et al. J. Am. Chem Soc., 2002, 124, 124, 12418). These acid purification steps oxidize the sidewalls of the CNTs at defect sites. This highly-aggressive oxidation typically shortens the CNTs from several microns down between 0.1 to 1 μm. This is a disadvantage. Short CNTs are less useful for applications based on their length or aspect ratio. For example, long length is advantageous in an epoxy type system where the starting products are monomeric. The longer CNTs have an increased ability to transfer load to more entanglements.